Separate names with a comma.
Discussion in 'Amplifiers' started by RunAway15, Apr 25, 2017.
So can someone explain the problems I may run into when building an HG amp from Toshiba plans.
Not the same electrical parameters. Will tune different. At least on the 2879. Not sure on the 2290's. Single driver 1446 also tuned a lot different. I mean a lot. Did a single 2879 HG as well, it was a totally different animal as well.
I only know, because I keep getting so called newer builders amps in and they are not even close. Had to redo circuits and figure out designs for each.
I might just scratch the building idea. Thanks for the reply. I appreciate it.
Grab a pair of matched non-red dot Toshibas @
[RFpartsDOTcom], build your self a 2-pill?
This way you will be 95% there with just the old-school blue-print!
That's a pretty good chunk of coin just for the transistors, I imagine building an amp from the ground up will be very costly and time consuming, but ultimately a very rewarding experience!
I see the BLF188XR selling for 185$, and it has huge output power compared to two 2879's.
What does the red dot stand for? And matched? I see that all the time.
Red Dots were a later version that conformed to RoHS Eurotrash requirements of No-Lead. I understand they may not take high temperatures as well as the non-Red Dot Toshibas.
From RF Parts:
" The following information applies only to the Toshiba 2SC2879 & 2SC2979A.
Early design Toshiba 2SC2879 do not have the small red dot on the package (lower right). In contrast, the later “A” version has the dot to identify that it uses a
different insulator between the chip and the flange base.
The 2SC2879A meets all specifications on the Toshiba Data Sheet, however, we believe
the heat conductivity from chip to flange with the original 2SC2879 part is better.
Also, we found that the 2SC2879 had a bit higher Collector to Base breakdown voltage,
which would be of interest it the transistor was to be pushed beyond factory specs."
Matched means they hopefully have been checked for similar gain specs.
Thanks for that info.
Knock on wood...I've never had a Toshiba 2sc2290 or 2sc2879 transistor fail. Red dot or not.
Keep in mind that is when they were run within or just slightly above their design parameters and keeping IMD (distortion/ harmonics) within acceptable low levels
I have had only two Toshiba 2SC2879 transistors fail over the years. And it was completely my (stupid) fault. I wasn't paying attention and installed them both 180 degrees off. I have not made that mistake again..........
A matched pair of transistors has a pair of transistors that have the same beta gain or close so they work well together in a push-pull circuit. Imagine you had two engines in a car and you wanted one to power the right side of the car and one to power the left side of the car. If each engine was not producing the same HP and TQ or they where totally independent of each other and you stepped on the gas the car would not want to pull straight. Whichever engine was putting out the most power or was revving slightly higher than the other would pull the car in towards that more power engine.
In an amp with more than one transistor if one is has a higher beta much higher beta gain than it's partner they will not work well together and they will fail in short order. Only one transistor is working at a time as one switches on and saturates the other is resting when the first transistor saturates it turns off and the partner takes over. Imagine you and a friend have to keep a flywheel spinning for hours at a time and it has remain smooth and close to the same speed constantly if you and our partner are the same height and weight and similar in strength once you both get the timing down right the two of you can keep that flywheel spinning for a long time and at close to a constant speed something neither of you could do alone. Likewise, if one guy was 6 foot 9 240lbs and the other guy was 5 foot 8 and 150lbs. it is going to be much harder for them to work well as a team.
If you force two dissimilar transistors to work together it will work for a short time then it will fail so we match the electrical properties of like transistors so they will work well together. With bjt's we do this in pairs. If you need 4 transistors each set of 2 needs to be a match. Ideally, we want each set of two to be close to each other and to the next pair. TO make balancing idiot proof we will sometimes go for matched quads or more but you do not need to have all for matching so long as each set of 2 is matching.
I over simiplified it but I hope it helps!
i run hg 2879 on volts and so far no problems i run 8 pill been using this for comp for come weeks now no problems.
Ideally you want every transistor and tuning capacitor in the amp matched. If you only match the pairs that make up each section the difference in power output will be dissipated by the balancing resistors in the output combiner.
This is why you see some poorly built amps burning the 100 ohm resistors on the output combiner. People band aid that with higher wattage resistors. If everything is right you can test it with 1/4 watt resistors on the output combiner. Hammer on it for a few minutes and the resistors should still be cool. If all is good put the 2 watt resistors back on and call it a day.
I have been saying that for a couple of years now.
I have a matched pair of HG's but I have yet to build with them. As long as I have Mot transistors and Toshiba I will use those first. Never volt any transistor beyond the datasheet unless you just like behaving in a very foolish manner. You might as well order products from Acme and take lessons from "Wilde Coyote"(sp). Stay inside the data sheet numbers and always check your SWR before you start jaw jacking and you will have transistors that last for decades many times over. I have never blown a final of any kind in a radio or amplifier and I have been into CB since 1990 big time. On the other hand, I have known men that could not go 3 months without blowing something!
THe thing with MOT and Toshiba is quality control and consistency! HG while the best Chinese transistors I have seen they are still wild cards when it comes to durability and reliability. On top of that the average gain on Toshibas is normally around 85db but with HG almost always when I see them matched they are reading around 65db for beta gain. The HG's I got from HG are like wise marked 65.